1. Field of the Invention
This invention relates generally to the measurement of optical power and more particularly to measurement of the gain (or quench ratio) of photonic inverting devices.
2. Description of the Related Art
Photonic integrated circuits (PICs) require inverting logic gates with gain greater than one to enable logic functionality with fanout and cascade. A photonic inverter is a core component within photonic inverting logic gates. Optically modulated semiconductor laser technology is speed-limited by the photon cavity lifetime and, therefore, can provide high speed, integrable, photonic inverters that can be used as digital logic gates. If semiconductor lasers can be configured as photonic inverters with gain exceeding one, then photonic clocks, oscillators and logic gates would be possible at high data rates.
Elements necessary for a viable high-speed photonic logic family are: Boolean completeness, gain greater than one for cascade and fanout, sufficient modulation depth to drive successive logic devices, and a process technology compatible with circuit components and established design tools. From De Morgan""s laws, multiple-input inverting gates are sufficient to form a Boolean complete logic family. In an article entitled xe2x80x9cOperation Characteristics of A Side-Light-Injection Multiple-Quantum-Well Bistable Laser for All Optical Switchingxe2x80x9d by Hiroyuki Uenohara et al., published in the Jpn. J. Appl. Phys., vol. 33, pp. 815-821, 1994, a semiconductor laser was demonstrated as a photonic inverter gate using the gain quenching effect, where the output amplitude of the logic laser is suppressed (quenched) when an input signal is present. In this article, input-power data and output-power data are presented that can be used to calculate gain for a photonic inverter based on a semiconductor laser with a side-injection input configuration. The input power measurement documented in this article is measured external to the photonic inverter input port, and neglects insertion losses that degrade the input signal as it propagates from the input port to the active volume of the semiconductor laser where the gain quenching occurs. The method presented in this article for measuring the input power data does not measure the input power responsible for the gain quenching observed. This article does not disclose the present invention. A semiconductor laser inverter with two inputs has the capacity to perform a logical NAND or NOR function.
When developing or evaluating photonic inverter structures, there is a requirement to measure the minimum gain of the photonic inverters as a metric to quantify performance changes of devices as a function of engineering changes between devices or operational parameters. When the devices under measurement are photonic inverters based upon semiconductor lasers with a side-injection input configuration, the physical geometries of the devices involved do not allow for the direct measurement of the internal photonic input signal power responsible for the quenching function of the photonic inverter. One can easily measure the input power incident upon the device input interface, but to design optimal photonic inverter devices, one needs to quantify the input optical power injected within the active volume of the photonic inverter semiconductor laser cavity which is solely responsible for the gain quenching modulation of the laser output power.
The actual power coupled into the laser active region of a photonic inverting device will be less than the total incident input power because of losses associated with scattering, absorption, and transmission. External incident power could be measured with any number of standard optical power measurement techniques but as the associated losses are unknown, the actual power coupled into the photonic inverter laser active region cannot be determined from this measurement. It is this actual power responsible for the quenching interaction that must be accurately characterized for practical design of photonic inverters. Heretofore, prior demonstrations of such measurements have not explicitly considered these losses and have made measurements of the input power external to the actual device at the device input interface. Knowledge of the actual power responsible for the quenching of the photonic inverter is required to optimize the design of the actual physical parameters of the photonic inverter device.
In consideration of the problems detailed above and the limitations enumerated in the partial solutions thereto, an object of the present invention is to provide a method for measuring the optical power injected within the active volume of a side-injection input photonic inverter.
Another object of the present invention is to provide a measurement method that does not include errors introduced by various insertion losses.
Yet another object of the present invention is to provide a method that yields a value for maximum injected optical input power that can be used to evaluate the minimum gain associated with operation of a photonic inverter.
In order to attain the objectives described above, according to an aspect of the present invention, there is provided a method for measuring gain of a photonic inverter whereby the input optical signal power is determined by the measured photocurrent induced by the side-injected input optical signal, and the output power of the photonic inverter is measured by standard techniques.
The present invention is a method to measure gain of a side-injection input photonic inverter based on a semiconductor laser using two different modes of operation. In one mode, the device is operated as a photonic inverter device and in the other mode as a photogenerated current measurement device.
While the device is operated in a photonic inverter mode, that is, pumped at a magnitude that supports photonic inverter operation, the optical output power is measured in the absence of an input signal and with an input signal that quenches the output of the photonic inverter. While the device is operated as a photogenerated current measurement device with an input optical signal, a reverse bias is applied to offset any forward bias induced by the injected input optical signal, the induced photocurrent is measured, and the wavelength of the input optical signal is measured. From these measurements, the gain is calculated using the difference in the photonic inverter optical output power with and without the optical input signal while operating in photonic inverter mode and the input power responsible for the quench of the photonic inverter while in photogenerated current measurement mode.